Microwave Engineering Notes Questions and Answers part 2

Tuesday, 12 November 2013

Microwave Engineering Notes Questions and Answers part 2


Here i am sharing two marks microwave engineering notes part-2

51. What is transit time?

          The time taken by an electron to travel from the cathode to the anode plate of an electron tube is known as transit time.

52. Write the classification of microwave tubes.
          They are classified into two types.

          1. O-type microwave tube or linear beam

          2. M- type microwave tube

53. What do you mean by O-type tubes? Name some O-type tubes.

In O – type tube a magnetic field whose axis coincides with that electron beam is used to hold the beam together as it travels the length of the tube. It is also called as linear beam tube.

i) Helix traveling wave tube

ii) Coupled cavity TWT

iii) Forward wave amplifier

iv) Backward wave amplifier

v) Backward wave oscillator

54. Define velocity modulation.

          The variation in electron velocity in the drift space is known as velocity modulation.

55. What do you mean by klystron?

          A klystron is a vacuum tube that can be used either as a generator or as an amplifier of power at microwave frequencies operated by the principles of velocity and current modulation.

56. Mention the application of two-cavity klystron.

          1. Used in Troposphere scatter transmitters.

          2. Satellite communication ground stations.

          3. Used in UHF TV transmitters.

          4. Radar transmitters.

57. Define Transit time in Reflex klystron.

The time taken by the electron to travel into the repeller space and back to the gap.

58. What are the high frequency effects in conventional tubes?

The high frequency effects in conventional tubes are

i) Circuit reactance

a) Inter electrode capacitance

b) Lead inductance

          ii) Transit time effect

          iii) Cathode emission

iv) Plate heat dissipation area

v) Power loss due to skin effect, radiation and dielectric loss.

59. What are the assumptions for calculation of RF power in Reflex Klystron?

i) Cavity grids and repeller is plane parallel and very large in extent.

ii) No RF field is excited in repeller space

iii) Electrons are not intercepted by the cavity anode grid.

iv) No debunching takes place in repeller space.

v) The cavity RF gap voltage amplitude is small compared to the dc beam voltage.

60. Give the drawbacks of klystron amplifiers.

1. As the oscillator frequency changes then resonator frequency also changes and the feedback path phase shift must be readjusted for a positive feedback.

2. The multicavity klystron amplifiers suffer from the noise caused because bunching is never complete and electrons arrive at random at catcher cavity. Hence it is not used in receivers.

61. What is the effect of transit time?

There are two effects.

1) At low frequencies, the grid and anode signals are no longer 180o out of phase, thus causing design problems with feedback in oscillators.

2) The grid begins to take power from the driving source and the power is absorbed even when the grid is negatively biased.

62. What are the applications of reflex klystron?

1) Signal source in MW generator

2) Local oscillators in receivers

3) It is used in FM oscillator in low power MW links.

4) In parametric amplifier as pump source.

63. Give the performance Specification of Reflex klystron?

Frequency range: 2- 200 GHz

Band width: + 30 MHz

Power output: 10 mW – 2.5W

Efficiency: 20 to 30%

64. What is TWTA?

          A Traveling Wave Tube Amplifier (TWTA) circuit uses a helix slow wave non-resonant microwave guiding structure. It is a broadband device.

65. What is the purpose of slow wave structures used in TWT amplifiers?

Slow wave structures are special circuits that are used in microwave tubes to reduce wave velocity in a certain direction so that the electron beam and the signal wave can interact. In TWT, since the beam can be accelerated only to velocities that are about a fraction of the velocity of light, slow wave structures are used.

66. How are spurious oscillations generated in TWT amplifier? State the method to suppress it.

In a TWT, adjacent turns of the helix are so close to each other and hence oscillations are likely to occur. To prevent these spurious signals some form of attenuator is placed near the input end of the tube which absorb the oscillations.

67. State the applications of TWT.

1) Low power, low noise TWT’s used in radar and microwave receivers

2) Laboratory instruments

3) Drivers for more powerful tubes

4) Medium and high power CWTWT’S are used for communication and radar.

68. What are the advantages of TWT?

1. Bandwidth is large.

2. High reliability

3. High gain

4. Constant Performance in space

5. Higher duty cycle

69. What are the applications of klystron amplifier?

(1) UHF TV Transmitters

(2) Long ranger radar

(3) Linear particle accelerator

(4) Troposcatter links

(5) Earth station transmitter.

70. Name four types of slow wave structures.

          1. Helical line.

          2. Folded back line.

          3. Zigzag line and

          4. Interdigital line.

71. Why magnetron is called as cross filed device?

          In magnetron, the dc magnetic field and dc electric field are perpendicular to each other and hence magnetron is called as a cross filed device.

72. What are the types of magnetron?

          1. Split anode magnetron

          2. Cyclotron-frequency magnetron

          3. Traveling wave magnetron

73. State the applications of magnetrons.

          1. Radar transmitters

          2. Industrial heating

          3. Microwave ovens.

74. What is frequency pulling and frequency pushing in magnetrons?

Frequency pulling is caused by changes in the load impedance reflected into the cavity resonators.

Frequency pushing is due to the change in anode voltage which alters

the orbital velocity of electron clouds.

75. State the characteristics of magnetron and of 2-cavity klystron amplifier.

Magnetron:

Operating frequencies - 70 GH z

Output power - 40 MW

Efficiency - 40 to 70%

2-cavity klystron:

Efficiency - 40%

Power output -> 500 KW

Pulsed power-_> 30 MW

Power gain- about 30 dB.

76. What is strip line?

          The strip line consists of a central conductor called strip and two ground plates. The dominant mode in strip line is TEM.

77. Define microstrip line.

          The microstrip line consists of a conductor strip and a ground plane. The electromagnetic wave propagates in quasi TEM mode.

78. Mention the types of losses in microstrip line.

          1. Dielectric loss

          2. Ohmic loss and

          3. Radiation loss

79. What is parallel strip line?

          The parallel strip line consists of two perfectly parallel strips separated by a perfect dielectric slab of uniform thickness.

80. What do you mean by coplanar strip line?

          The coplanar strip line consists of two conducting strips on one substrate surface with one strip grounded.

81. Define partially shielded strip line.

          A partially shielded strip line has its strip conductor embedded in a dielectric medium, and its top and bottom ground planes have no connection.

82. Why conventional open wire lines are not suitable for microwave transmission?

          The conventional open wire transmission lines are not suitable for microwave transmission due to high radiation losses that are associated when the wavelength becomes smaller than the physical lengths of the conventional lines at high frequency.

83. Define planar transmission.

          A planar transmission line is a transmission line with conducting metal strips that lie entirely in parallel lines.

84. Write the advantages of microstrip lines.

          1. The microstrip lines have a power handling capacity of a few watts which is quite adequate for most microwave circuits.

          2. With the advantage of low loss, high electric constant materials, microstrip has become popular, particularly in the fabrication of microwave integrated circuits.

          3. Microstrip lines are used to interconnect high-speed logic circuits in digital circuits.

85. Define attenuation constant.

          The sum of dielectric and ohmic losses may be expressed as losses per unit length in terms of an attenuation constant.

          The attenuation constant α = αd+αc

Where αd is the dielectric attenuation constant and

                                         αc is the ohmic attenuation constant.

86. What are the advantages of coplanar strip line over conventional parallel strip line?

          1. The two strips of coplanar strip lines are on the same substrate for convenient connections.

          2. It eliminates the difficulties involved in connecting the active and passive circuit components in shunt from the conducting strip to the ground plane on the same side of the substrate.

          3. Reliability is increased than conventional strip line.

87. Write the classification of electronic circuits.

          Electronic circuits are broadly classified into three categories based on the circuit technology.

1.     Discrete circuit

2.     Integrated circuit and

3.     Monolithic Microwave Integrated Circuit(MMIC)

88. What do you mean by discrete circuit?

          The circuit elements are separately manufactured and then interconnected by conducting wires is now referred to as discrete circuit.

89. Define IC.

          The IC consists of a single crystal chip of semiconductor typically 50 x 50 mils in cross section containing both active and passive elements and their interconnection.

90. What are the advantages of MMICs over discrete circuits?

          MMICs offer the following advantages over discrete circuits.

1.     Small size and weight.

2.     High reliability.

3.     Improved reproducibility.

4.     Improved performance and

5.     Eventual cost reduction when produced in large quantities.

91. Mention the materials used in MMICs.

          The basic materials for monolithic microwave integrated circuits are broadly divided into four categories.

1.     Substrate materials.

2.     Conductor materials.

3.     Dielectric materialsand

4.     Resistive materials.

92. What is called as film integrated circuit?

          An MMIC whose elements are formed on an insulating substrate, such as glass or ceramic, is called a film integrated circuit.

93. What is the need for dielectric materials?

          Dielectric materials are used in monolithic microwave integrated circuits for blockers, capacitors and some couple-line structures.

94. What is the need of resistive materials?

          Resistive materials are used in monolithic microwave integrated circuits for bias networks, terminations and attenuators.

95. Write some of the properties of resistive materials.

          1. Good stability.

          2. Low temperature coefficient of resistance.

          3. Adequate dissipation capability.

96. Why monolithic technology is not well suitable for microwave integrated circuits?

          Monolithic technology is not well suited for microwave integrated circuits because the processing difficulties, low yields and poor performance have seriously limited their applications.

97. What is the need of diffusion and ion implantation?

          Diffusion and ion implantation are the two processes used in controlling amounts of dopants in semiconductor fabrications.

98. Write the advantages of ion-implantation method?

          1. Precise control of the total amount of dopants.

          2. The improvement of reproducibility and

          3. Reduced processing temperature.

99. What is lithography?

          Lithography is the process of transferring patterns of geometric shapes on a mask to a thin layer of radiation sensitive material, which is known as resist, for covering the surface of a semiconductor wafer.

100. Name the different types of lithography.

          1. Electron beam lithography.

          2. Ion-beam lithography.

          3. Optical lithography and

          4. X-ray lithography.

101. What do you mean by slotted line?

          Slotted line is a fundamental tool for microwave measurements. Slotted line consists of a section of waveguide or co-axial line with a longitudinal slot. This slot is roughly 1 mm wide and allows an electric field probe to enter the waveguide for measurement of the relative magnitude of field at the location of the probe.

102. Define reflection co-efficient.

          The ratio of electric field strength of reflected and incident wave is called reflection co-efficient.

103. What is voltage standing wave ratio?

          Voltage standing wave ratio is defined as the ratio of maximum voltage to the minimum voltage.

104. Define return loss.

          The return loss is a measure of the power reflected by a line or network or device.

105. Mention the drawback in return loss measurements.

          1. The unstability of the signal source causes a change of signal power level during the measurement of input and reflected signal levels at different instants of time.

          2. Non-ideal directional couplers and detectors are also cause error.

106. Define power.

          Power is defined as the quantity of energy dissipated or stored per unit time.

107. What are the methods to detect microwave power?

          1. Bolometer and

          2. Calorimeter method.

108. What is Bolometer?

It is a power sensor whose resistance change with changed temperature as it absorb the microwave power. It is a short thin metallic wire sensor with positive temperature coefficient of resistance.

109. What is calorimeter?

It is convenient device setup for measuring the high power at microwave which involves conversion of microwave energy in to heat, absorbing the heat in a fluid and determine the temp.

110. Mention the sensors used for microwave power measurements.

          The sensors used for microwave power measurements are the Schottky barrier diode, Bolometer and the Thermocouples whose resistance changes with the applied power.

111. What is a VSWR meter?

VSWR meter is a highly sensitive, high gain, high theta, low noise voltage amplifier tuned normally at fixed frequency of 1KHZ of which microwave signals modulated. This meter indicates calibrated VSWR reading for any loads.

112. What is calorimetric direct heating method?

          In the calorimetric direct heating method, the rate of production of heat can be measured by observing the rise in the temperature of the dissipating medium.

113. What is calorimetric direct heating method?

          In the calorimetric indirect heating method, heat is transferred to another medium before measurement.

114. List the different types of Impedance measurement methods?

1. Slotted line method

2. Reflectometer method

115. What do you meant by reflection loss?

          The reflection loss is a measure of power loss during transmission due to the reflection of the signal as a result of impedance mismatch.

116. Define insertion loss?

The insertion loss is a measure of the loss of energy in transmission through a line or device compared to direct delivery of energy without the line or device.

117. What are the contributions of insertion loss?

          The insertion loss is contributed by

                    1. Mismatch loss at the input

                    2. Attenuation loss through the device.

                    3. Mismatch loss at the output

118. How do you measure microwave frequency?

1. Wavemeter method

2. Slotted line method

3. Downconversion method

119. What is a wave meter?

It is a device used for frequency measurement in microwave. It has cylindrical cavity with a variable short circuit termination .It changes the resonant frequency of cavity by changing cavity length.

120. Define scattering parameters.

          Scattering parameters are defined as the ratio of the outgoing waves to the incident waves. The incident and reflected amplitudes of microwaves at any port are used to characterize a microwave circuit.

121. Define dielectric constant?

It is defined by the ratio of permittivity of medium to permittivity of free space.

122. What do you meant by isolation?

          The isolation between E and H arms are defined as the ratio of the power supplied by the generator connected to the E-arm (port 4) to the power detected at H-arm (port 3) when side arms 1 and 2 are terminated in matched load.

123. List the methods for measuring dielectric constants?

1. Waveguide method

2. Cavity pertubation method

124. What are classifications of power measurements?

          The classifications of power measurements are

1.     Low power (less than 10 mW)

2.     Medium power (from 10 mW to 10 W) and

3.     High power (>10 W)

125. Distinguish between low frequency measurements and microwave measurements.


Low frequency measurements Microwave measurements
At low frequency it is convenient to measure
voltage and current and use them to calculate
power.
At microwave frequencies the amplitudes of the
voltages and current on a transmission line are the
functions of a distance and are not easily measurable.
At low frequency, circuits use lumped elements. At microwave frequencies, the circuit elements
are distributed.
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